Filter is a kind of optical element with selective absorption of different bands of light. It is made of plastics or polished and coated glass. Generally, red filters can only pass through red light. The transmittance rate of glass is almost the same with that of air, so all coloured light can pass through. But after dyeing, the molecular structure of glass will change, the refractive index will change either, which influence the transmission of certain coloured light. For example, when a white light passes through a blue filter, it will emit a blue light, while the green light and red light are very few, because they are absorbed by the blue filter.